1. Field of the Invention
The present invention relates to robots controlled by a computer system in order to urge a tool, integral with a robot's arm, to follow a memorized path.
2. Discussion of the Related Art
Many robots are capable of moving a tool, such as milling cutters, torch welders, etc., within a prememorized precise path. In order to move a tool according to n degrees of freedom, a robot must have at least n axes. A degree of freedom corresponds to a translation or rotation movement in accordance with an axis of a three-dimensional referential system. With 6 degrees of freedom, all the translation and rotation combinations of a tool are possible (the tool can be positioned anywhere, according to any orientation within a determined range of action of the robot). The relation between the articulation variables (rotation angles of the robot's axes) and the position and orientation of the tool is given by: EQU P=f(A)
where P is a vector whose components represent the position coordinates x.sub.i (i=1 . . . 3) and orientation coordinates r.sub.i (i=1 . . . 3) of the tool, A is a vector whose components represent the articulation variables a.sub.j (j=1 . . . 6), and f is a vector function.
A slight variation dA of the articulation variables about vector A is then associated with a slight variation dP of the position vector by the relation EQU dP=[J(A)]dA,
where [J(A)] is a Jacobian matrix whose coefficients are functions of the components of vector A.
Considering the accuracy of trajectories obtained with robots, it would seem advantageous to use them in the surgical field. However, when the present robots are operating, they exhibit hazards that render them unsuitable for surgical treatments. Indeed, in case of failure of the control circuit of the robot's motor, or simply because of power supply trouble, such as a brief current interruption, the robot could make an uncontrolled movement of large magnitude and high power that would endanger a patient withstanding a surgical act determined by the robot.
Nowadays, robots are used in the surgical field only for prepositioning and are blocked during the surgical act. For example, a neuro-surgical act consists in inserting a needle into the brain at a very precise spot. To achieve this purpose, one uses a robot that positions a cylinder guiding a needle in a very precise way with respect to the patient's head, using a system allowing to detect the position of the head. Once the guiding cylinder is positioned (far enough from the patient so that he cannot be reached by an uncontrolled movement of the robot), the robot's axes are blocked and its power supply is turned off. Then, the surgeon inserts the needle in the desired way into the guiding cylinder. Thus, in the surgical field, the use of robots is presently limited to the detection of a precise initial position.